The invention relates to an inching braking system including an inching valve assembly and a brake valve assembly in accordance with the preamble of claim 1.
The like inching braking system are utilised in hydraulically operated work tools, for example in wheel loaders, stacker trucks etc., wherein the hydraulic drive system is also utilised for decelerating, besides the brake system proper. Braking with the aid of the drive system is referred to as xe2x80x9cinchingxe2x80x9d. Splitting the braking force between brake system proper and drive system has the advantage that wear to the brakes of the work tool may be reduced quite considerably in comparison with solutions lacking an inching braking system inasmuch as the drive system may primarily be utilised for deceleration at minor changes of speed.
In the like conventional systems, actuation is achieved, for example, through a brake pedal, through which the brake system of the work tool receives application of a braking pressure in dependence on the actuation angle of the pedal. This braking pressure acts as a control pressure for an inching valve, through which the drive system of the work tool may be controlled for inching. The brake system is biased by means of a return spring or a biasing valve in such a way that the brake will take effect only after a minimum braking pressure of, e.g., 9 bar has been exceeded. In such conventional solutions, the characteristic line for the braking pressure rises relatively shallowly in the inching range to then receive a greater gradient when the inching pressure drops to zero, so that the actual braking process takes place subsequently to the inching process. The inching pressure will thus reach its minimum value when the braking pressure enters into the range wherein the braking process is initiated through the brake unit.
Upon use of such conventional inching braking system it is found that particularly in the transitional range from the inching process to the introductory phase of the actual braking process discontinuities may occur, so that the inching process is already overlapped by the onsetting braking process. Inasmuch as these discontinuities are not manageable in the conventional systems, it was proposed in DE 195 25 582 A1 to this applicant to couple the brake valve assembly and the inching valve assembly of an inching braking system via common transfer means, so that the spools of both valve assemblies are controlled upon actuation of the brake pedal. Owing to this measure it is ensured that the braking pressure for the brake system will rise only once the inching pressure has dropped to zero. In this way, the discontinuities in the transitional range from inching to braking are prevented.
In some applications it may, however be advantageous if the braking pressure assumes a predetermined threshold value already during the inching process. This specific pressure management of the inching braking system having intersecting inching and braking characteristic lines is not possible in a defined manner with the aid of the above described constructions.
In contrast, the invention is based on the object of creating an inching braking system wherein the braking pressure may be built up substantially independently of the inching pressure.
This object is attained by an inching braking system having the features of claim 1.
Owing to the measure of directly coupling the spool of an inching valve assembly through a driving member with the actuation means, preferably with the brake pedal, and associating with the brake piston an actuator which will effect an actuation movement of the brake piston only after a predetermined stroke of the driving member, the brake piston may be taken into its controlled position when maximum inching, i.e., the maximum braking effect due to the drive system, has not been reached yet. In this manner, accurate harmonisation of the inching and braking processes is possible, wherein different regions of intersection of the characteristic lines of the inching valve assembly and of the brake valve assembly may be adjusted by means of a corresponding design of driving member and actuator. In this way, the entire braking process (inching and braking through the brake system) may optimally be adapted to the respective operating conditions.
The structure of the inching braking system is particularly simple if the driving member for the inching valve spool is formed as pin means which contact the actuation means on the one hand and the inching piston on the other hand.
The space required by the inching braking system may be minimised if the two pistons of the braking and inching valve assemblies are accommodated inside a housing such as to have a successive coaxial arrangement, and the pin means slidingly extend through the brake piston so that the latter, as it were, serves as a guide for the pin means.
The structure of the actuation means is particularly simple if they have the form of an actuation rod biased into its rest position by an inching spring acting on an inching spring plate, with the pin means also contacting the inching spring plate.
In order to compensate offset between the centers of the brake piston and a guide bush for the inching piston, the pin means are preferably constituted by two pins having a successive coaxial arrangement, one of which is guided within the brake piston and the other one within the guide bush.
In a preferred embodiment, the brake piston carries on one end portion a spring plate for supporting a brake spring, wherein the end portion of the brake spring which is removed from the spring plate, may be taken into contact with the actuation rod.
In order to ensure against breakage of the inching spring, the actuation rod may be provided with a stop which may be taken into contact with the inching spring plate.
Advantageously, the inching piston and the pin means are biased through an inching pressure spring into their basic positions, with the brake piston being supported through a control spring on the adjacent end portion of the guide bush for the inching piston.
Further advantageous developments of the invention are the subject matters of the further subclaims.